Variable gain circuit



Dec. 17, 1968 G. J. OVERTVELD 3,417,340

VARIABLE GAIN CIRCUIT Filed March 4, 1965 4 Sheets-Sheet 1 L l 6 e I00loo ATTENUATOR 1 lo '29 E db Loss F I62 I? I FIGS |ooo L f UA IOO-INVENTOR G. J. OVERTVELD AGENT 1968 G. J. OVERTVELD 3,417,340

VARIABLE GAIN CIRCUIT Filed March 4, 1965 4vShee'ts-Sheet z HARMONIC 3RD2RD DISTORTION E0 oi'z345s775-mm N HARMONIC 3, 2, DISTORTION INVENTOR G.J. OVERTVELD AGENT Dec. 17, 1968 G. J. OVERTVELD 3,417,340

VARIABLE GAIN CIRCUIT Filed March 4, 1965 4 Sheets-Sheet 5 cF z Flew-INVENTOR G. .l.' OVERTVELD AGENT 17, 1968 G. J. OVERTVELD 3,417,340

VARIABLE GAIN CIRCUIT Filed March 4, 1965 4 Sheets-Sheet 4 INVENTOR s..l. OVERTVELD AGENT ilnited States Patent U 3,417,340 VARIABLE GAINCIRCUIT Gilles Jozias Overtveld, Ottawa, Ontario, Canada. assignor toNorthern Electric Company Limited, Montreal, Quebec, Canada Filed Mar.4, 1965, Ser. No. 437,137 7 Claims. (Cl. 330-24) ABSTRACT OF THEDISCLOSURE The invention relates to a transistorized variable gaincircuit comprising a transistor and a junction diode connected in serieswith the emitter-collector electrodes of the transistor. A source ofinput signal is applied to the base electrode of the transistor and asource of control voltage for controlling the AC. impedance of the diodeis connected to the emitter electrode. The output of the variable gaincircuit is taken from across the diode and may be varied in accordancewith the control current applied to the diode. Because the outputimpedance of the transistor is high attenuations of 20 db per decade ofcontrol current have been obtained. Also a fairly high control currentthrough the diode can be obtained readily with small values of controlvoltage.

This invention relates to controlled attenuators and more particularlyto variable gain circuits capable of controlling the level of an audioprogram automatically.

Speech or music levels are subject to large variations that do oftenoccur unexpectedly. It is technically impractical to design the audiosystem between the loudspeaker of the listener and the studio microphoneso that it would accommodate the total dynamic range of all possibleaudio levels. It is therefore necessary to manipulate attenuators duringthe course of an audio program to prevent over modulation and distortionon high or excessive noise on low audio levels.

This need for control of program level has resulted in the developmentof a variety of attenuation devices which are inserted in the linkbetween the studio microphone and the listeners loudspeaker. Some of theknown devices are the light-sensitive resistors, the Hall effect devicesand the Variable Mu tubes.

Most of these devices, however, do not satisfy all the main criteria forsuitable variable gain circuits which are: (1) Maximum variable gain fora given distortion (cg.

40 db range of input signal).

(2) Low distortion.

(3) Uniformity of control between devices.

(4) The speed at which gain can be changed (attack time).

(5) Effect on frequency response.

The light-sensitive resistors for example have low distortion at highsignal level because light not current is the actuating source. Howeverthe variation between two individual light-sensitive resistors does notseem to be much better than 2 db. Non-uniformity of control makes themunsuitable for stereo channel separation where 0.5 db is required or ascalibrated remote attenuator. Furthermore the incandescent light bulb isslow and the neon light source too irregular to make the light sensitiveresistor suitable for automatic volume control or as a peak limiter inaudio. Speeds of attack smaller than 50 #866. are also desirable inthese applications while the light sensitive resistor at their best haveattack times measured in milliseconds.

The Hall effect device has proved to be unsuitable because ofnon-uniformity of control between devices and because, of sensitivity totemperature variations.

Variable gain tubes generate unwanted transients be- 3,417,34ti PatentedDec. 17, 1968 cause once balanced they will, due to aging or temperaturevariation, soon become unbalanced.

One way of overcoming the variation in control characteristics is byusing the variable gain circuit in a feedback control loop. The systemoutput level then determines the loss. This loss will be uniformregardless of the difference in transfer function between devices. Thisis why variable mu tubes are used extensively in feedback control loopsfor peak limiter applications. But for many other applications afeedback control loop cannot be used e.g. whenever the control is notderived from the level of the controlled signal itself. For instance,the left channel of stereo may be desired to be controlled byLeft-t-Right. It is therefore important that control of the variablegain device can be obtained over an extended range (0-40 db loss) whichis uniform with respect to control current or voltage from device todevice while maintaining all other characteristics such as distortion,speed of response, and effect on frequency response.

Furthermore it has been found that in practice the desired constantoutput cannot be obtained with a feedback system because the magnitudeof feedback is limited by stability. Where a large control range isrequired, the high frequency response and attack time are sacrified toobtain stability. Where a short attack time is necessary the highfrequency response is increased and the control range reduced. In otherwords constant output and attack time are interdependent. Feedbackdesign becomes further complicated by generation of unwanted controltransients which decay according to loop stability and cause overcorrection.

From the above it is seen that the feedback control is not satisfactoryto overcome the deficiencies of the prior art devices. The forward feedcontrol offers more advantages because with the forward feed control itis possible to combine fast attack time and a fiat output over a wideoperating range and because over correction is not possible. Furthermorethe forward feed control makes it possible to control the output bymeans of an independent control signal.

Another basic attenuator circuit comprising a ratio circuit including ahigh impedance element in series with a junction diode is also found inthe art. The input signal is applied to the full ratio circuit while theoutput signal is taken across the junction diode. A source of controlvoltage supplies a control current to the junction diode which variesthe small signal A.C. current resistance of the diode inversely inaccordance with the control current thereby varying the output of theattenuator inversely in accordance with the control voltage. Thiscircuit may be used with advantage in a forward feed control because thecontrol voltage may be derived from the input signal or from anindependent source.

Furthermore experiments have revealed that the the silicon junctiondiodes have an excellent uniformity in dynamic impedance. It has beenfound that in a total of about diodes over a range of 40 db less than0.2 db variation could be expected.

In practice however, it was found that the above circuit does not havevsufficient attenuation. To obtain the desired attenuation an attenuatorcircuit was developed in accordance with the invention. The circuitcomprises a transistor having its emitter and collector electrodesconnected in series with a junction diode. The input signal is appliedto the base or emitter electrodes and the output is taken across thejunction diode. A source of control voltage which may be derived fromthe input. signal or from an independent source supplies a controlcurrent to the junction diode through the emitter collector electrodesof the transistor and thereby varies the small signal A.C. currentresistance of its diode in very much the same way as the above mentionedratio circuit.

The invention wil now be described with reference to the drawings inwhich:

FIG. 1 is a block diagram of an attenuator whose output is controlled bya control voltage;

FIG. 2 illustrates the transfer characteristic of an attenuator having aloss of 20 db/DC of increase in input signal;

FIG. 3 illustrates the circuit diagram of a basic attenuator circuit;

FIG. 4 illustrates the transfer characteristic of the attenuator shownin FIGURE 3;

FIG. 5 illustrates an attenuator circuit in accordance with theinvention;

FIG. 6 illustrates the harmonic distortion versus signal across thediode shown in FIG. 5 for a constant current;

FIG. 7 illustrates the harmonic distortion versus control current acrossthe diode shown in FIGURE 5 for a constant signal voltage;

FIG. 8 illustrates a second embodiment of the invention;

FIG. 9 illustrates a third embodiment of the invention;

FIG. 10 illustrates the complete transfer function of the attenuatorshown in FIGURE 9 in dbm versus dbm and FIG. 11 illustrates a fourthembodiment of the invention.

In FIGURE 1, there is shown a block diagram of a variable gain circuitof the open loop type comprising an attenuator L whose transfer functionis controlled by a DC. control voltage E which may be derived from stantoutput. This is exemplified in FIGURE 2 which illustrates the desiredcharacteristic of an attenuator having a db loss per decade of increaseof input signal. In this FIGURE 20 log e /e is plotted versus the inputvoltage e on a semilog scale.

In FIG. 3, there is shown a basic attenuator circuit comprising an A.C.source of signal e a DC. source of control voltage E, a resistor R and ajunction diode D across which is taken the output of the attenuator. Thejunction diode has a good characteristic for application in a gaincontrol circuit because the small signal resistance of a junction diodebiased in the forward direction may be expressed analytically at:

or the small signal resistance is proportional to the inverse of I whereK is a constant. Now if in FIGURE 3 the small signal current isproportional to e then the small signal output is:

which means that by varying the DC. control voltage E, the gain of theattenuator may be controlled accordingly.

In FIGURE 4, the loss of suchi an attenuator 20 log e /e is plottedversus the control current I on a semilog scale with R as a parameter.If R is large (200K) the loss approaches 20 db per decade of I as seenfrom the drawing. It would therefore be possible to use the circuit ofFIGURE 3 as a 20 db/dec. attenuator. However if R equals 200K ohms, Ewill have to be fairly high in order to provide an adequate currentthrough I the diode which means that in practice the above circuitcannot be used for attenuations of 20 db/dec. and higher.

In order to obtain higher attenuations a circuit such as shown in FIGURE5 was developed in accordance with the invention. This circuit comprisesa transistor Q wherein the signal voltage 2 is applied to its base andthe control voltage applied to the emitter through a resistor R. Ajunction diode D is connected in the collector circuit 4 of transistor Qand the output 2 is taken across the diode D. The collector biasingpotential is provided by source Vcc.

Because the output impedance of the transistor Q is high adequateattenuation can be readily obtained. Furthermore a fairly high controlcurrent I through the diode can be obtained readily with small values ofE. With this circuit attenuation up to 60 db/dec. can be obtained. Withp.21. initial current and R=500 ohms the insertion loss is 0 db.

Another factor that governs the feasibility of variable gain devices isthe amount of harmonic distortion. The harmonic distortion versus signalacross the junction diode is shown in FIGURE 6 for constant controlcurrent of 30 a. Distortion versus control current with constant signalis shown in FIGURE 7. These measurements suggest that real improvementcan be obtained by balancing as per FIGURE 8.

In FIGURE 8, the AC. signal is applied out of phase to the two bases oftransistors Q and Q A source of control voltage E is applied to thejunction point between resistors R and R which are connected to theemitters of transistors Q and Q respectively. The biasing potential ofthe collector electrodes is provided by a source cf DC. potential Vcc.The oppositely poled diodes D and D are connected in series with thecollector electrodes of transistors Q and Q and the ouptut signal istaken across both diodes. The second harmonic cancels out in thiscircuit leaving the third harmonic which is approximately A of thesecond harmonic. Furthermore the distortion decreases with increasing Ias shown in FIGURE 7. From the curve, it is seen that I should be keptlarger than 50 a.

A further extension of the circuit of FIGURE 8 is shown in FIGURE 9. Thecircuit of FIGURE 9 is identical to the circuit of FIGURE 8 except forthe addition of transistor Q The addition of transistor Q transforms thecircuit in a so-called long-tail pair. In this circuit the total currentis fixed by E/R The division of current between Q and Q is not onlydependent on R ad R but also on the V of each transistor which as weknow vary with the temperature.

A change in temperature of Q and Q by current or environmental effectsof temperature effects both transistors, hence the current ratio betweenthe two transistors Q and Q is equal to and is not disturbed by anychange in temperature.

FIGURE 10 illustrates the transfer characteristic of the attenuatorshown in FIGURE 9 in dbm versus dbm If E is proportional to e then itbecomes possible to adjust the transfer curve for flat, decreasing orincreasing output with input by varying R This is possible because thecontrol current may be changed by varying R without affecting the gainwhich is determined by A further embodiment of the invention is thecircuit of FIGURE 11. The AC. signal e is applied to the bases oftransistors Q and Q through transformer T The collector circuit oftransistors Q and Q includes resistors R R and R; which determines theinput impedance of the attenuator.

A control voltage :2 which may be proportional to e or an independentcontrol signalis applied to the base of a transistor Q through arectifier Re and capacitor C. The amplitude of the rectified voltageapplied to transistor Q and the value of variable resistor R in theemitter circuit of transistor Q determines the amount of control currentwhich will flow in transistors Q and Q and diodes D and D Resistor Rprovides for proper balance of the control currents in transistors Q andQ Oppositely poled diodes D and D are connected across the collectorelectrodes of transistors Q and Q The signal output across thecollectors is roughly the ratio of the small signal resistance of D andD and the sum of R R and R This signal is directly coupled totransistors Q and Q which serve as balanced emitter followers andisolate the variable output impedance of the diodes from the outputtransformer T which is connected between the emitter electrodes oftransistors Q and Q Potential sources V V and V and resistors R R R andR provide for proper biasing of transistors Q Q Q and Q; as it is wellknown in the art. Variable resistor R also provides for proper balancingof transistors Q and Q It is very important to note, in connection withthe disclosed attenuator circuit that the control current nowhere passesthrough reactive elements and that consequently this circuit can changegain in a few ,usecs.

While in the disclosed attenuator circuit, the signal source is appliedto the base electrode of the transistor, it is understood that thesignal source may be connected to the emitter electrode in series withthe control voltage source. In such a case however a control currentequal to I as compared to I /B in the disclosed circuit passes throughthe signal source e. In the case where the signal source is atransformer, the energy stored in the transformer will be higher andeffect the response characteristic of the attenuator.

What is claimed is:

1. An attenuator circuit for producing an amplification gain varyingproportionally to the magnitude of a control voltage comprising atransistor having base, emitter and collector electrodes, an inputcircuit connected to the base and emitter electrodes of the transistor,and including an input signal source and a control voltage source,connected to said input circuit and a junction diode across which istaken the output signal connected in series with the emitter andcollector electrodes of the transistor through said input circuit, thecathode of said diode being connected to said collector electrode forforward conduction of said diode; the control voltage supplying acontrol current to the junction diode through the emitter and collectorelectrodes of the transistor for controlling the small signal A.C.resistance of the diode inversely in accordance with said controlcurrent thereby varying the output of said attenuator inversely inaccord- 4. An attenuator circuit as defined in claim 1 wherein saidcontrol voltage source is controlled from an independent source ofcontrol.

5. A balanced attenuator circuit .for producing an amplification gainvarying proportionally to the magnitude of a control voltage comprisingtwo transistors connected back to back each transistor having base,collector and emitter electrodes, an input signal being applied in phaseopposition to the base electrodes of the transistors, a source ofcontrol DC. voltage connected to the emitter electrodes and a pair ofoppositely poled junction diodes across which is taken the outputsignal, the cathode of said junction diodes being connected to thecollector electrodes, the source of control voltage supplying a controldirect current to said diodes through the emitter and collectorelectrodes of the transistors for controlling the small signal AC.current resistance of the diodes inversely in accordance with saidcontrol current thereby varying the output of said attenuator inverselyin accordance With said control voltage.

6. A balanced attenuator circuit for producing an amplification gainvarying proportionally to the magnitude of a control voltage comprisingtwo transistors connected back to back each transistor having base,collector and emitter electrodes, an input signal being applied in phaseopposition to the base electrodes of the transistors, a source ofcontrol DC. voltage connected to the emitter electrodes, a pair ofoppositely poled junction diodes across which is taken the output signalconnected across the collector electrodes, the source of control voltagesupplying a control direct current to said diodes for controlling thesmall signal A.C. current resistance of the diodes inversely inaccordance with said control current thereby varying the output of saidattenuator inversely in accordance with said control voltage, and athird transistor, said source of control voltage being applied to theemitter electrodes through said third transistor whereby the amount ofcontrol current is controlled by said third transistor.

7. An attenuator circuit as defined in claim 6 further including abalanced emitter follower stage connected to the output diodes forisolating the variable output impedance of the diodes from the output ofthe attenuator.

References Cited UNITED STATES PATENTS 3,015,781 1/1962 Eklov 330-243,127,577 3/1964 Lapointe 33029 ROY LAKE, Primary Examiner.

LAWRENCE J. DAHL, Assistant Examiner.

US. Cl. X.R. 33029

